This comes straight from Insomniac's brochure released on 1ts January,2008.
Summary of SPU usage found on page 34 of the 38 paged documentation :-
SPUs
All our systems started off as RAW mode
The only long term (not finished this frame) asynchronous
processing is the collision on the raw SPUs
We use [Job Manager] but not all systems use it in the typical way
of fire and forget. Most of our system require the SPU to be
running a particular system at the same time as the PPU.
To ensure the SPU is doing what we want at the correct time we
send [Job Manager] the job and use our own thin
synchronization and job buffering schemes using the locked-line
for communication
10-20% total SPU utilization
